Why the Induction Motor Could Be the Better Choice for Your Electric Vehicle Program

Home , Induction motor, Synchronous motor

EVS26 Los Angeles, California, May 6-9, 2012

Why the Induction Motor could be the better choice for your electric vehicle program

Derrick Zechmair, Kurt Steidl Siemens, Erlangen Germany, [email protected] Siemens, Erlangen Germany, [email protected]

Abstract Low weight and high power density is a main focus in electro mobility. The most common traction motors for vehicle applications are synchronous motors or induction motors whereas synchronous motors are preferred because of their advantage regarding power density and weight. But each of the motor concepts has its advantages and disadvantages. In this paper it is shown that Siemens has managed to increase the power density of induction motors and simultaneously reduced the main disadvantage of the induction motor by optimizing some parts of the motor. With this adaption it is possible to reduce the weight and as a result of this defining parameter the induction motor has in summary more advantages compared with the synchronous motor. Therefore it can be the better choice for electro mobility

Keywords: synchronous motor, induction motor, power density, efficiency

stator interacts with the rotary field of 1 Introduction the rotor, this results in torque. The motor shaft rotates. The use of permanent magnetic synchronous Permanent magnetic Synchronous Motors (PSM) motors has not only advantages. The magnets are are normally designed as inner magnet systems for made of rare earth material and this material electric vehicle use. In this case synchronous increased tremendously in costs and also needs motors build the magnetic flux with permanent higher efforts for safety measures. To take into magnets and for that reason an external account this issues an optimized induction motor magnetising current for the rotor is not necessary. by eliminating the disadvantages can be the Thus the losses in the rotor are minimized till alternative. nominal speed. Hence, the motor can be designed more compact and lighter. When operating with field weakening as a result of 2 Details the permanent magnet construction a field 2.1 Functionality of a synchronous weakening current has to be implanted. This motor (Fig. 1) current influences the efficiency of the motor.

When the stator winding is energized with 3 phase voltage, it causes a rotating current and a magnetic field: The magnets of the rotor have a permanent magnetic field. The rotary field of the

Permanent Stator behaviour. This increases the power density but Magnets however there is a difference compared with Stator permanent magnetic synchronous motors. Winding Advantageously, on the other hand there are the

lower costs and the higher robustness to be Rotor mentioned.

Squirrel Cage Stator Fig. 1 Shaft Stator

Winding Fig. 2 shows the flow of the magnetic field lines on an example of an 8-pole synchronous motor. Rotor As it is shown in the illustration the magnetic flux is acting mainly on the outer diameter of the rotor. The inner diameter and shaft are mostly field-free and therefore not coactive necessary Shaft for the magnetic field. This matter leaves room Fig. 3 for weight optimization in this area. Fig. 4 shows the flow of the magnetic field lines on an example of a 2-pole induction motor. Stator creates the magnetic field of the rotor Higher power losses in the rotor Stator Hot rotor Permanent Less current than rated at high speed and part-load Magnet Stator The induction motor needs the iron in the rotor to Rotor Winding build a magnetic field for generating a torque in Magnetic Field the air gap. Weight reducing action can not be Lines Current Shaft Carrying done as it can be done with a synchronous rotor. Stator 8 Pole Motor Winding Fig. 2 Squirrel Stator cage

2.2 Functionality of an induction motor Current Carrying Magnetic Field (Fig. 3) When the stator winding is energized Lines with 3 phase voltage, it causes a current and builds up a magnetic field. The Stator rotary field Shaft Rotor 2 Pole Motor induces voltage into the squirrel cage of the Fig. 4 rotor. The induced voltage of the rotor causes current and torque. The rotary field of the stator interacts with the rotary field of the rotor. This 3 Cost aspect results in torque. The motor shaft rotates. Synchronous motors use rare earths for magnets, prices for rare earths skyrocketed during the last Induction Motors induce the magnetic field years. Rare earths share of total value add for within the rotor by implanting a short current in motor increased from 6% to 41% for electric motor the squirrel cage. This concept effects higher suppliers between 2010 and 2011 (average). losses in the rotor and causes higher heat and less efficiency. Depending on the material of the squirrel cage the losses are more or less higher. Standard low voltage motors use squirrel cages made of aluminium. Higher efficiency motors use copper because of its better electromagnetic

- high induced voltage when operating as generator 500 USD/kg - safety aspects of hazardous material (magnets) 450 higher efforts in manufacturing, service, 400 recycling 350 - identification of the rotor position necessary 300 - higher system costs 250 200 4.2 Induction motor Advantages: 150 + high max. speed, high range of field weakening +2,328% 100 + low current at no load and part load operation 50 + no rare earths necessary 0 + robust design 05 06 07 08 09 10 11 12 13 + no hazardous material, Fig. 5 easy recycling Fig.5 shows the trend of prices of Neodymium + high safety with low effort from 2005 till today + low production costs

3,500 USD/kg Disadvantages:

3,000 - small torque density higher weight 2,500 bigger volume - high current at constant torque 2,000 - high power losses in the rotor hot rotor 1,500 +4,608%

1,000

500 5 Siemens Innovations 0 05 06 07 08 09 10 11 12 13 Fig. 6 5.1 Innovativ rotor manufacturing

Lamination Fig. 6 shows the trend of prices of Dysprosium stack from 2005 till today

4 Comparison of the motor concepts 4.1 Permanent synchronous motor Advantages: + high torque density + high continuous torque + high efficiency Copper rod + low power losses cool rotor temperatur up to rated power Short circuit ring + wide range of constant power Fig. 7 Disadvantages - high costs because of rare earth magnets Short circuit ring and connection with the copper - efforts in field weakening necessary rod made of die cast aluminum Fig. 7 (field weakening current)

Innovativ cooling system

Terminal Siemens Innovations Box

Housing Stator

Windings Further Optimizations - Adaption of rotation speed up to 20,000 rpm - Power/torque optimization for real worl driving cycles Rotor

Elimination of the most disadvantages of the induction motor Shaft Cheaper motor with high efficiency

Bearing shields Fig. 8 Authors Cooler rotor with opimized cooling Higher power density Derrick Zechmair……..

Results: With optimized material and suitable cooling technique the power density and efficiency can be almost the same as with synchronous motors. The main disadvantages such as weight and installation space are comparable to synchronous motors and the induction motor gains in importance as a technical similar concept by a huge cost benefit. Kurt Steidl received his Polytechnic degree in Micromechanics from the Nürnberg University of Applied 6 Conclusion Sciences, Germany in 1991. Kurt is currently the Product Portfolio Manager for motors in Under normal conditions, with optimized the Electric Vehicle performance and because of the better cost Infrastructure business unit at position the induction motor be the appropriate Siemens in Erlangen, Germany. solution for electro mobility.

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